RATIONALE: Monoclonal antibodies, such as trastuzumab, can block tumor growth in different ways. Some block the ability of tumor cells to grow and spread. Others find tumor cells and help kill them or carry tumor-killing substances to them. It is not yet known whether giving trastuzumab for 6 months is as effective as giving trastuzumab for 1 year in treating patients with breast cancer.

PURPOSE: This randomized phase III trial is studying trastuzumab to see how well it works when given for 6 months compared to 12 months in treating women with nonmetastatic breast cancer that can be removed by surgery.

Arm I (control arm, 12-month therapy): Patients continue to receive trastuzumab (Herceptin®) IV until a total of 12 months of therapy is completed in the absence of disease progression or unacceptable toxicity.

Arm II (experimental arm, 6-month therapy): Patients continue to receive trastuzumab* IV until a total of 6 months of therapy is completed in the absence of disease progression or unacceptable toxicity.

NOTE: *Patients who have already finished 6 months of trastuzumab at randomization do not receive further trastuzumab.